There have been heretofore many nitride semiconductor light emitting elements, such as LEDs (light emitting diodes) and semiconductor lasers, in which sapphire is used as a substrate and a light emitting element structure including a plurality of nitride semiconductor layers is formed on the substrate of sapphire by epitaxial growth. The nitride semiconductor layer is represented by the general formula: Al1-x-yGaxInyN (0≤x≤1, 0≤y≤1, 0≤x+y≤1).
The light emitting structure has a double-hetero structure in which an active layer including a nitride semiconductor layer of single-quantum-well structure (SQW) or multi-quantum-well structure (MQW) is sandwiched between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. When the active layer is an AlGaN-based semiconductor layer, by adjusting an AlN molar fraction (also referred to as an Al composition ratio), band gap energy can be adjusted within a range where band gap energies that can be taken by GaN and AlN (about 3.4 eV and about 6.2 eV) are lower and upper limits, so that an ultraviolet light emitting element having a light emission wavelength of about 200 nm to about 365 nm is obtained.
A nitride semiconductor light emitting element is formed as follows. A plurality of nitride semiconductor layers are epitaxially grown on a wafer substrate composed of sapphire or the like, and an electrode structure is formed. Thereafter, the wafer substrate is divided into chips each having a planarly-viewed shape of a square or a rectangle. In the case where a sapphire wafer substrate is employed, generally the thickness of the wafer is about 300 μm or greater. A thin wafer is so difficult to be handled in a process of forming the element structure on the wafer substrate, that the wafer is highly possibly damaged. On the other hand, the wafer is preferably thin for being diced along the boundary on the wafer between the chips with a diamond blade, a laser or the like so that the wafer is divided into individual chips. In general, the wafer has its back surface polished (which may include abrading, grinding and the like as pretreatment) to attain a reduced thickness of 100 μm±30 μm, more preferably 80 μm to 100 μm, and thereafter is divided into individual chips (for example, see Patent Documents 1 and 2 below).
In Patent Document 1, a sapphire substrate of a blue-color LED of 350 μm square being a nitride semiconductor light emitting element has its thickness reduced to 150 μm, preferably to 100 μm. Further, in Patent Document 2, a sapphire substrate of a blue-color LED of 300 μm to 500 μm square being a nitride semiconductor light emitting element has its thickness reduced to 50 μm to 120 μm, preferably to 80 μm to 100 μm.
Meanwhile, since light emitted from the active layer travels in every direction, that is, toward both the upper and lower surfaces of the chip, there exists a back-surface-emitting type nitride semiconductor light emitting element in which the light traveling toward the upper surface is reflected by the upper surface of the chip downward, so that the reflected light together with the light traveling toward the lower surface is transmitted through the sapphire substrate and output from the lower surface of the chip to the outside of the chip (for example, see Patent Documents 3 and 4 below).